The present invention relates to an optical semiconductor device such as a light emitting semiconductor device and a light receiving semiconductor device, and more particularly to an optical semiconductor device that is excellent in reliability.
A semiconductor laser device of a background art will be described with reference to FIGS. 1 to 3. In this example, FIG. 1 is a perspective view showing a semiconductor laser device. FIGS. 2 and 3 are cross-sectional views showing a semiconductor laser device.
Referring to FIG. 1, in the semiconductor laser device, electrode capacitance adjustment PADs 30 are formed on the right and left sides of a stripe 60 including an active layer formed on an InP substrate 10. After a passivation layer 40 not shown has been formed, through-holes are formed on the stripe 60. Further, after an electrode layer 20 has been formed, the electrode layer 20 is photo-etched to form a p-electrode at a right side of FIG. 1. Although not shown in the figure, the n-electrode is formed on a rear surface (lower surface) of the InP substrate 10 after the rear surface of the InP substrate 10 has been polished.
An electrode at the left side of FIG. 1 is of a float potential and is formed in order to facilitate the handling of the semiconductor laser device. Also, laser oscillates at a lower portion of the stripe 60, and is output toward an anteroposterior direction. The output ratio is determined by reflective layers not shown which are disposed on the end surfaces.
FIG. 2 is a cross-sectional view taken along a broken line in FIG. 1. In the present specification, the cross section is not hatched. This is because the complication of the drawing is prevented. As described in FIG. 1, the PAD 30 is formed on the lower portion of the p-electrode and the float electrode. The passivation layer 40 covers the entire active surface of the semiconductor laser device, and is opened above a mesa 50 that is a part of the stripe 60 to obtain an electric connection between the mesa 50 and the electrode layer 20.
FIG. 3 is a cross-sectional view showing a hithermost portion (output terminal) of FIG. 1. In the semiconductor laser device, the electrode layer 20 covers the entire surface of the stripe 60 at the output terminal. Electrode layer terminals 20b are formed at both side positions apart from the mesa 50 having a width of about 1 μm by several to ten μm by ion milling. The positions of the electrode layer terminals 20b are designed from their capacitance. In this example, each of the electrode layers 20 is a deposited layer made of Ti/Pt/Au, and its thickness is 150 nm/40 nm/750 nm. Also, Ti is for securing adhesion with a substrate, Pt is for stopping the diffusion of Au into the substrate, and Au is for a current layer and wire bonding. A stress develops in the electrode layer 20 when the layer grows, and remains in the layer. The stress becomes maximum at the electrode layer ends 20b which are discontinuous points. The thickness of the InP substrate 10 whose rear surface has been polished is 90 μm which is thin. On the other hand, the stress of the electrode layer 20 is a tension stress of GPa order. As a result, the stress of the electrode layer 20 distorts the InP substrate 10 in a convex shape (the electrode layer surface is inside) downward. In the present specification, the description of Ti/Pt/Au means that Ti is the lowest (InP substrate side), and Au is the uppermost.
According to the inventors' study, the distortion affects the distribution of electrons and holes below the mesa 50 to be thickened or thinned in the longitudinal direction (a vertical direction on the paper plane). As a result, the current density when laser oscillates is increased, and crystal may be destroyed by concentration of a developed heat. Since the semiconductor laser device oscillates the laser in a state of a laser module attached to a heat sink, the distortion is corrected. However, it is necessary to take the measures against the crystal destruction.